That gurgling sound in your sink means there is an issue with the air in the drain pipes. Air normally is supplied through the vents, so either the vents are blocked, or they weren't correctly installed. If the gurgling happens only when you use the sink, you probably should focus on the sink vents, but if it happens when you flush a toilet or drain the tub, you need to examine the entire vent system.

The Gurgling Sound

When you open a sink drain or flush a toilet, a head of water enters the waste pipe. Since it fills the pipe, the water pushes air in front of it and creates a vacuum behind. The vacuum exerts considerable internal force on the pipes. Unless you provide a vent so more air can come in and fill it, the air will enter through any available opening -- usually a sink, shower or tub P-trap. The gurgling sound you hear from your sink is the sound of air rushing past the water standing in the sink or in the bottom of the trap.

Troubleshooting the Sink Vent

If the gurgling happens only when you drain the sink, there's probably a problem with the venting for that sink. The vent may not have been installed the correct distance from the P-trap, or it may be blocked. For 1 1/2-inch drain pipes, the vent should be installed within 3 1/2 feet of the bottom of the trap. If it's farther than that, it's likely a vacuum is developing and causing the gurgling. Your drain may have an air admittance valve instead of a vent; it's usually situated on the back of or underneath the sink. Check the operation of this valve, as it may be stuck.

Troubleshooting the Main Vent

When the main vents are blocked, flushing a toilet or draining a bathtub can produce gurgling sounds at a sink in a different bathroom. When this happens, it's a warning to clean the vents before the vacuum gets strong enough to drain the sink trap and fill the house with sewer odor. You clear the main vent from the roof, either by spraying water into the vent opening, which usually is just above the main bathroom, or using a sewer auger. The auger needs to be long enough to reach the sewer, because the blockage could be anywhere between there and the vent opening.

Considerations

Since water flows slowly when there isn't enough air in the waste pipes, sediment and debris can settle, and blockages are more common. Consequently, you may find yourself frequently cleaning out the P-trap, snaking the pipes and pouring down drain cleaner, only to have to do it again in a couple of months. If you heed the message of the gurgling sound, however, you can save all these efforts by taking care of venting issues. The free movement of air through the pipes restores the natural flow of water, and that washes waste into the sewer.

About the Author

Chris Deziel has a bachelor's degree in physics and a master's degree in humanities. Besides having an abiding interest in popular science, Deziel has been active in the building and home design trades since 1975. As a landscape builder, he helped establish two gardening companies.